A discharge lamp having a high mercury vapor pressure is used for a light source of a projector device. In such a high-voltage mercury lamp, the light in a visible wavelength region can be obtained with a high output by making the mercury vapor pressure high.
The discharge lamp has a light emitting part which is formed by a discharge vessel and has a substantially spherical shape. A pair of electrodes is arranged in the light emitting part so as to be opposite to each other with an extremely small distance, for example, 2 mm or less.
In the case that the discharge lamp described above is lighted in the same state for a long period, a plurality of minute protrusions may be formed or minute irregularities may be generated in the tip surface of the electrodes due to a high temperature. The minute protrusions and the irregularities are generated by the melting of a material (for example, tungsten) constituting the electrodes and the aggregation of a chemical compound generated by being bonded to gas which is sealed within the light emitting part, and this existence changes the shape of the tip surface of the electrodes. It has been known that a starting point of an arc moves in conjunction with this, the discharge position becomes unstable, and there has been a problem that a flickering of a projection light called as a flicker is generated.
In order to solve such problems, Patent Document 1 discloses a technique for inputting into a lamp an alternating current in which an amount of input current is increased at a predetermined timing. FIG. 13 is a view showing an example of the conventional lamp current waveform. A current pulse P shown in FIG. 13 is structured so as to include a pulse P1 which is input to the discharge lamp at a normal time, and a pulse P2 in which the amount of the input current is increased at a predetermined timing, compared to the pulse P1.
Since the pulse P2 is intermittently inserted, a degree of heat applied to the electrode is enhanced at this timing, and it is possible to transmit the heat not only to an electrode tip but also to a position distant from the tip. Therefore, the heat is transmitted to the position which is distant from the electrode tip during this time, and it is possible to melt and evaporate minute protrusions and irregularities which are generated at the position. As a result, it is possible to eliminate the protrusions and irregularities at the other positions than the electrode tip which may affect adversely, and it is possible to stabilize a luminescent spot of the arc.